Multiplexed direct current regulation output circuits are one kind of driving electric source for many types of electronic devices. Multiplexed direct current regulation output circuits have been widely used in various electronic products such as liquid crystal displays, personal computers (PCs), notebooks, and the like. For example, a liquid crystal display generally includes a backlight module, a liquid crystal panel, and a multiplexed direct current regulation output circuit. The multiplexed direct current regulation output circuit includes a high voltage output for driving the backlight module, and a low voltage output for driving the liquid crystal panel.
Referring to FIG. 3, a typical multiplexed direct current regulation output circuit 1 includes a transformer 10, a power control chip 11, a feedback circuit 12, a sampling circuit 19, a first half wave rectifier circuit 13, a second half wave rectifier circuit 14, a first filter circuit 15, a second filter circuit 16, a first output 17, and a second output 18.
The sampling circuit 19 includes a first resistor 191, a second resistor 192, a third resistor 193, and a feedback node 194. One end of each of the first, second and third resistors 191, 192, 193 is connected to the feedback circuit 12 via the feedback node 194. The other end of the first resistor 191 is connected to the first output 17. The other end of the second resistor 192 is connected to the second output 18. The other end of the third resistor 193 is connected to ground. The sampling circuit 19 is configured to sample voltages of the first and second outputs 17, 18. The feedback node 194 is configured to provide a mixed sampling voltage to the feedback circuit 12. When the mixed sampling voltage is equal to 2.5 volts, the feedback circuit 12 does not work. When the mixed sampling voltage is greater than or less than 2.5 volts, the feedback circuit 12 works. The first resistor 191 has a lower resistance, preferably 12 KΩ (kiloohms). The second resistor 192 has a greater resistance, preferably 91 KΩ. The third resistor 193 has a resistance approximately equal to 8 KΩ.
The first half wave rectifier circuit 13 includes a first rectifier diode 132, and a first resistance-capacitance (RC) series circuit 131 connected with the first rectifier diode 132 in parallel. The anode of the first rectifier diode 132 is connected to the transformer 10. The cathode of the first rectifier diode 132 is connected to the first filter circuit 15.
The second half wave rectifier circuit 14 includes a second rectifier diode 142, and a second RC series circuit 141 connected with the second rectifier diode 142 in parallel. The anode of the second rectifier diode 142 is connected to the transformer 10. The cathode of the second rectifier diode 142 is connected to the second filter circuit 16.
The transformer 10 provides voltages to the first output 17 via the first half wave rectifier circuit 13 and the first filter circuit 15 in series, and further provides voltages to the second output 18 via the second half wave rectifier circuit 14 and the second filter circuit 16 in series. The first output 17 is configured to output a low direct current voltage, e.g., 5 volts. The second output 18 is configured to output a high direct current voltage, e.g., 18 volts.
The feedback circuit 12 is configured to provide changes in the mixed sampling voltage of the sampling circuit 19 to the power control chip 11. The power control chip 11 is configured to adjust a pulse duty ratio provided to the transformer 10 according to each change in the mixed sampling voltage, so as to adjust the output of the transformer 10.
When the multiplexed direct current regulation output circuit 1 is used in a liquid crystal display, the first output 17 provides the 5 volt voltage to a liquid crystal panel driving circuit of the liquid crystal display, and the second output 18 provides the 18 volt voltage to a backlight driving circuit of the liquid crystal display.
When the liquid crystal display is turned on, the first output 17 is loaded by the liquid crystal panel driving circuit. Thus, the voltage at the first output 17 drops to approximately 4.3 volts. Moreover, a voltage difference between the anode and the cathode of the first rectifier diode 132 rises slightly, such that the voltage at the first output 17 further drops to approximately 4 volts. At the same time, because the backlight driving circuit is not yet turned on, the second output 18 is not loaded. Therefore the 18 volt voltage at the second output 18 is maintained.
Because the voltage at the first output 17 drops to 4 volts, a current flowing through the first resistor 191 is decreased, while a current flowing through the second resistor 192 remains the same. According to Kirchhoff's electrical current law, a current flowing through the third resistor 193 is decreased, such that the mixed sampling voltage is decreased below 2.5 volts. In such case, the voltage provided from the feedback circuit 12 to the power control chip 11 is decreased, such that the power control chip 11 increases the pulse duty ratio provided to the transformer 10. Thus the voltages at the first output 17 and the second output 18 are increased.
When the voltage at the first output 17 is increased to 4.5 volts, the voltage at the second output 18 is increased to 28 volts, and thus the currents flowing through the first resistor 191 and the second resistor 192 are increased. Accordingly, the current flowing through the third resistor 193 is increased, and the mixed sampling voltage at the feedback node 194 is increased to 2.5 volts, which makes the feedback circuit 12 stop working. However, the voltage at the first output 17 is only 4.5 volts, which is less than the working voltage of 5 volts needed for the liquid crystal panel driving circuit. Therefore the liquid crystal display employing the multiplexed direct current regulation output circuit 1 may not work normally. Thus, the multiplexed direct current regulation output circuit 1 may have low reliability.
What is needed, therefore, is a multiplexed direct current regulation output circuit that can overcome the above-described deficiencies.